Ever hear of a Mersenne prime? These are prime numbers that are one less than a power of two. Named after Marin Mersenne, a French Minim friar, who studied them in the early 17th century, there is a distributed computing project on the Internet to find Mersenne primes called GIMPS (Great Internet Mersenne Prime Search). The project recently announced they have found the largest known prime.
The number in question is 274,207,281-1 and with 22,338,618 digits it is almost five million digits longer than the previous record holder. The number is too big to be useful for modern cryptography. Prime95, the software that searches for these unique primes, is often used by overclockers as a CPU stress test (the software has a stress mode to make sure the prime tests are accurate). It was also responsible for uncovering a bug in the Intel Skylake CPU.